As shown very diagrammatically in perspective in FIG. 1 on the attached drawings, an aircraft engine comprises a central part 1 in which the engine itself is housed, and an annular part 2 called the nacelle surrounding the central part of the engine coaxially and delimiting an annular duct 3, called the fan duct, with it. A fan, driven by the central part 1 of the engine, is located in this duct 3 at the fan cowls.
The front part of the nacelle 2 forms an air intake structure 4. This structure has the particular function of controlling aerodynamic air flow firstly towards the fan duct 3, and secondly towards the outside of the nacelle 2. This type of structure normally comprises an air intake lip 5 (see FIG. 2) that is added onto the inner casing 6 and the outer casing 7 of the nacelle 2. By convention, if not mentioned otherwise, the terms “inner” and “outer” will be used to denote the position or orientation of parts with respect to the fan duct 3 of the air intake structure 4.
Normally, the inner casing 6, i.e. the side of the fan duct 3 at the aft end of the air intake lip 5, is composed of a sound proofing panel. This panel has its own structure capable of attenuating noise produced by the central part of the engine, and particularly by the fan. In practice, this structure is normally of the composite sandwich type, in other words the panel 6 comprises a honeycomb core.
The air intake lip 5 has a cross section in the shape of a U open in the aft direction. It forms the outer casing of the forward part of the air intake structure 4. It shares air between the part that penetrates into the fan duct and the part that flows around the nacelle 2.
A forward stiffening frame 8 and an aft stiffening frame 9 can pass through the structure between the inner and outer casings, in order to stiffen the structure 4. By convention, the terms “forward” and “aft” will be used throughout this text with reference to the forward and aft direction of the engine.
The forward stiffener frame 8 is placed inside and towards the aft of the air intake lip 5, at the intake end of the annular part formed by the casings 6 and 7. The aft stiffening frame 9 is placed inside the nacelle 2 close to the fan cowls, forward from the engine attachment area; the engine attachment area is located at an attachment area between the air intake and the fan cowls, but actually outside the sound proofing panel of the inner casing 6 for which it would destroy the characteristics. The function of the stiffening frames 8, 9 is to provide mechanical strength for the forward part of the nacelle and help preserve its shape and size. Consequently, they may be fixed using rivets or screws, for example to the nacelle 2, or for the forward stiffening frame 8 directly to the air intake lip 5.
Some examples of such an air intake structure are shown in American patent U.S. Pat. No. 6,328,258.
In flight, when the engine stops and starts to “windmill” due to the aircraft speed, a suction phenomenon occurs at the air intake 4 that draws the air intake lip 5 inwards towards the inside of the fan duct 3. Since the component parts of the nacelle are fixed to each other, the suction of the air intake lip 5 causes a deflection of each part from its normal position. This causes large clearances that degrade the aerodynamics of the nacelle as a whole and introduces a parasite drag. Furthermore, this deviation applies load to structure components, and forces generated by this marginal operating case have to be taken into account.
With the arrival of aircraft engines with a greater by-pass ratio and/or a greater fan diameter (for example engines for use on the heaviest wide body type aircraft), deflections applied to the air intake structure are greater than they were in the past. It is found that the forward stiffening frame 8 and the aft stiffening frame 9 cannot resist the deflections applied in new engines and that existing techniques do not provide a solution for overcoming this disadvantage.